1. Field of the Invention
The present invention relates to an abnormal heating prevention system for preventing abnormal heating of a thermal head used in a thermal printer and more particularly, to an embodiment of such an abnormal heating prevention system which is configured in the form of pure hardware to prevent the abnormal heating of a thermal head with a high reliability.
2. Description of the Related Art
Many facsimile machines or the like have conventionally been mounted with a thermal printer as its printing device. Such a thermal printer has had such a problem that, since the printer prints data on thermal sensitive paper or on ordinary paper through ink ribbon by electrically and directly heating an array of heating resistors of a thermal head, if an abnormal current flows through the heating resistors, then the thermal head or the recording paper is abnormally heated, which undesirably might lead to fire.
As a major cause of the problem, there may be considered that the runaway or the like of a microcomputer built in the thermal head as a printing controller causes a print-time setting signal to be maintained to be continuously active without any interruption, though the print-time setting signal is set to provide a heating drive time to the heating resistor array for each line of picture image and is set to normally be active for only each printing period of the signal.
To avoid this problem, there has been proposed an abnormal heating prevention system in which switch means for forcibly cutting off the power supply to the heating resistors is previously provided so as to be turned OFF when such an abnormally long active time of the print-time setting signal or an abnormally high thermal head temperature is detected, thereby preventing the abnormal heating of the thermal head.
The prior art system, however, has a difficulty that, since the aforementioned abnormal-heating preventing operation is carried out under control of the aforementioned printing control microcomputer, if a software program itself describing the processing procedure based on the microcomputer runs out of control, then the above remedy becomes useless. That is, even turning OFF the switch means becomes difficult and the positive prevention of the abnormal heating of the thermal head cannot be guaranteed.